1. Field of the Invention
The present invention is related to wind energy conversion systems and, more particularly, is directed towards a wind energy conversion system which is carried aloft by a lighter-than-air structure.
2. Description of the Prior Art
The advent of fossil fuel shortages has stimulated the development of alternative energy sources, and in certain regions of the world wind energy conversion systems (WECS) are becoming more efficient and competitive in generating large amounts of electricity for residential or commercial use. Commercial versions of a WECS traditionally consist of a wind-driven rotor coupled to an electrical generator which are mounted on a tower to raise the large diameter rotor off the ground and as high in the wind regime as possible.
The major challenges to a designer of a WECS are the dilute concentration of energy in the wind as well as the intermittent nature of the wind. The low power density of wind dictates that WECS of large size are required if sizable amounts of electrical power are to be generated. The intermittent nature of the wind normally results in the rendering of a WECS in an idle state much of the time. This has resulted in granting wind-generated electricity a value equal only to the fossil or nuclear fuel displaced with relatively little value granted for the capital equipment. This has seriously retarded commercial WECS development.
In addition, since the wind spectrum contains gusts and lulls, the stresses introduced into the rotor system of a WECS are large, and require rotor designs and support structures which are, to say the least, quite a challenge to the designer.
Since the power contained in the wind is a function of the cube of the velocity, the siting of a WECS becomes extremely important. Thus far, the best sites for a ground-based WECS have been on the coastlines in the northern hemisphere, as well as on mountain tops and hill tops. The latter elevations replace the high towers required to position the wind-driven rotor high enough to benefit from the velocity gradient of the wind. Unfortunately, the availability of prime high-altitude sites is severely limited, and although the cost of tower construction is greatly reduced for such sites, the expenses of road building, transporting the heavy components and subsequent erection of the WECS are high.
It is generally considered that WECS must be placed beyond the boundary layer portion of the wind in order to become inexpensive in terms of the energy yields. Additionally, a WECS must be designed to afford maximum protection against violent storms, which in the past have been primarily responsible for wind machine breakdown or destruction.
In addition to the inherent high cost attendant to the construction of a tower of sufficient height to position a large diameter (e.g., 300 foot) rotor, ground-based towers suffer from several other deficiencies. One deficiency is that there is a requirement for an open approach to the tower location, which leads to the desirability of utilizing higher altitude, but relatively inaccessible, bald hills for placement of a WECS tower. Further, construction of a tower necessarily results in a fixed height for the main rotor whose electrical generating capability is therefore at the mercy of the wind density at that particular height at any given point in time. Further, undesirable vibrations have been observed which result from what is referred to as "tower shadow" which occurs when the blade of the rotor passes adjacent to the tower and sets up a type of vibratory forcing function effect. Additionally, there are inherent energy losses due to tower drag, and it is difficult to erect, service and maintain the equipment positioned on the top of the tower.
Due to gravity loads, there presently exists a practical maximum limit for the rotor diameter of approximately 300 feet. Further, a WECS having a 300 foot diameter rotor that produces 2.5 megawatts of electricity presently costs approximately $3 million and includes a massive transmission, drive shaft and heavy bearings which add significantly to the cost of the WECS as well as complicate the tower design upon which such a massive system must be positioned. For example, a mechanical transmission required for a 300 foot diameter rotor would weigh approximately 150-200 tons.
In addition to the foregoing drawbacks, a conventional. WECS requires yaw motors, bull rings and the like, to turn the main rotor as the wind shifts direction in order to maintain effective orientation. Such yaw motors and associated controls are expensive for large diameter rotors, are very slow to react, and add to maintenance and servicing problems.
There is a type of WECS which is known to obviate the need for a mechanical transmission. Such a WECS is known in the art as an Enfield-Andreau wind machine (see page 18 of "Wind Machines" by Frank R. Eldridge, The Mitre Corporation, October 1975). The Enfield-Andreau wind machine operates on a depression principle wherein the blades of the propeller are hollow and are provided with apertures at their tips. Generally, the interior of the blades communicate through an air passage in the hub of the propeller with the outlet of an air turbine which is coupled to an electric generator. When the wind velocity is of a value sufficient to cause rotation of the propeller, the air within the hollow blades is induced, by reason of the centrifugal force generated by its own mass, to flow out through the apertures in the blade tips thereby forming a depression (i.e., a pressure lower than that of the surrounding atmosphere) within the hollow blades. The air within the air turbine is then at a higher pressure than that of the air remaining within the blades, therefore establishing a continuous flow of air through the air turbine, the hub, the interiors of the blades and out through the apertures at the tips. The flow of air through the air turbine supplies power to drive the electric generator. A typical Enfield-Andreau WECS is set forth, for example, in U.S. Pat. No. 2,784,556 to Perdue. Such a ground-based system, however, still requires the propeller hub to be capable of rotation about a vertical axis in order that it may face into the wind. Additionally, a conventional Enfield-Andreau WECS requires the incoming air to make at least three 90.degree. turns prior to expulsion through the propeller tips. Such a system inherently loses energy that it otherwise might have. Further, a ground-based Enfield-Andreau WECS suffers from the same deficiency set forth above with respect to other WECS, namely, the inability to take advantage of the high wind power densities found at considerable altitudes off the ground.
I am also aware of U.S. Pat. No. 4,073,516 which issued Feb. 14, 1978 to Kling. In this patent, the advantage of replacing a tower-based WECS with a gas-filled hollow body that carries a rotor assembly, current generator and alignment means is recognized. However, the apparatus disclosed in this patent for accomplishing these noteworthy objectives are complex. Initially, the system requires an alignment assembly for aligning the rotor to face into the wind, a ground anchor, and at least one captivating stay connecting the floating power plant to the anchor. The support body is connected to the captivating stay through a joint connection requiring three degrees of freedom. The rotors are gimbal-mounted at a variable relative position with respect to the support body but in fixed positions relative to one another. Additionally, the rotor assembly requires at least one pair of coaxially and coplanarly mounted counter-rotating rotors having their moments of momentum compensated. Again, while this patent does recognize the noteworthy advantage of elevating a wind-driven power plant into high-altitude winds by means of a lighter-than-air structure, the means for accomplishing same, it is felt, leaves much to be desired and may be impractical.
In my co-pending U.S. Patent Application, filed concurrently herewith and entitled "A LIGHTER THAN AIR WIND ENERGY CONVERSION SYSTEM", I set forth the combination of a lighter-than-air wind energy conversion system wherein the LTA envelope carries a main rotor and electrical generator to take advantage of high wind speeds available at high altitudes. The heavy mechanical transmissions of the prior art are eliminated by providing a hollow bladed main rotor which drives an induction turbine positioned within a substantially linear duct which is preferably located along the longitudinal axis of the LTA envelope. The apparatus set forth in said co-pending application takes advantage of and improves upon the depression turbine principles described above in connection with the Enfield-Andreau WECS.
There are, however, problems associated with depression turbines of the Enfield-Andreau type. The main problem is their low efficiency which results from the fact that hollow rotor blades are a poor pump due to jet damping, high velocities which cause pumping losses, and other losses resulting from the high velocity exit flow at the tips of the blades which is radial in direction. It would clearly be desirable if an alternate configuration could be advanced which allows separation of the air pump and the rotor system that would, in turn, allow individual optimization of the rotor system and air pump which communicates to the duct.
In addition to the foregoing, the hollow bladed rotor described in my co-pending application preferably utilizes untapered rotor blades due to reduced internal drag that would be high with tapered blades. However, approximately 40% more material is required to build an untapered blade of equal aerodynamic performance having the same diameter as a tapered blade, adding substantially to the cost and weight. Additionally, providing a twist to the blade, which is a costly manufacturing complexity for the hollow bladed rotor of my co-pending application, would be only one to two percent more efficient than an untwisted blade.
It is towards overcoming the above-noted deficiencies that the present invention is advanced.
I am also aware of the following U.S. patents which, together with the above-noted references, are considered by me to be the closest prior art to my invention: U.S. Pat. Nos. 1,717,552; 2,384,893; 2,433,344; and 3,936,652.